1. Field of the Invention
The present invention relates to a hydraulic excavator, such as a power shovel. More specifically, the invention relates to a hydraulic excavator apparatus including a boom, an arm, a bucket and a cylinder device, for pivotally driving the boom, the arm and bucket, in which the boom is formed with articulated first and second boom members.
2. Description of Related Art
Japanese Unexamined Patent Publication (Kokai) No. 64-66319 discloses a working machine of the type which the present invention is directed to. The disclosed construction of the working machine are illustrated in FIGS. 1 to 3. As can be seen, the tip end of a boom cylinder 1 is connected to a tip end of a first boom member 2a of first and second boom members 2a and 2b which form a boom 2.
This conventional apparatus operates in the following manner:
(1) First, while the tip end of a bucket 3 is engaged to a tip end portion of a frame 4, the boom cylinder 1 is expanded up to a maximum length, as shown in FIG. 2. This causes the first boom to be raised by a given amount. PA0 (2) An arm cylinder 5 is then expanded and tends to cause pivoting of the arm 6 toward a folded position about an articulation point of the second boom 2b. At this time, since the bucket 3 is engaged with the frame 4, the second boom 2b is shifted forward. Therefore, the first boom 2a is raised further. At a position where the first boom 2a has been raised sufficiently, a pin 7c is inserted through pin holes 7a and 7b of the first and second booms 2a and 2b for fixing them in the predetermined fixed relationship and to restrict further relative angular displacement with respect to one another. PA0 (3) Following this, a bucket cylinder 8 is operated to release the bucket 3 from the frame 4 and to establish an operating state, in which excavation can be performed by operating respective cylinders 1, 5 and 8.
In the above-mentioned conventional arrangement, it is necessary for a longer expansion length of the boom cylinder 1 during pivoting across a rising/falling dead point (on a line extending through a pivot point of the boom cylinder 1 and a pivot point of the first boom 2a) than when the boom is a fully raised excavating position.
Accordingly, at the operating position illustrated in FIG. 3, the boom cylinder 1 cannot assume a fully extended condition. Therefore, it is possible to cause erroneous operation wherein the boom cylinder is further expanded to place the boom 2 and the boom cylinder 1 at the rising/falling dead point. Under these conditions, the boom 2 is disabled and unable to be raised or lowered thus inhibiting excavating operation.
As an approach to this problem, it is possible to control the raising and lowering of the boom by means of a control system by sensing the boom position with a sensor. However, in general, it is difficult or even impossible to assuredly stop the boom at exactly the same point irrespective of the expansion speed (rate) of the boom cylinder. This makes the control by means of the control system ineffective and fails to prevent the above problem. In addition, using a sensor and a control system increases the complexity of the excavator resulting in reduced reliability and increased cost.